Pore size distribution is a crucial parameter which is required to evaluate reservoir producibility and rock quality for petrochemical applications. The only direct downhole representation of the pore size distribution is nuclear magnetic resonance (“NMR”) logging T2 distribution data. In petroleum engineering applications, the pore size distribution is in the size domain whereas the T2 distribution is in the time domain. A reliable estimate of surface relaxivity is required to convert time domain T2 data to size domain pore size distribution. The available techniques to estimate the surface relaxivity are in the form of correlations and do not apply in rocks with very fine grains such as shales.
Although T2 distribution is a representation of the pore size distribution, it is plotted in time domain rather than size domain. Surface relaxivity is the key parameter to convert NMR T2 distribution to size distribution. Direct measurement and calculation of surface relaxation is a challenging task for both conventional and unconventional rocks. The common methodology is to correlate pore/throat size distribution measurements such as mercury intrusion or nitrogen adsorption with NMR. Using this method, an estimate of surface relaxivity is calculated for downhole logging applications. The main disadvantage of this method is inability to assess the whole pore network due to pore accessibility limitations.